There are known internal combustion engines with mechanical tappet comprising elements commonly called "bucket type tappets" positioned between the cylinder head valves and the associated cams of the camshaft and having the purpose of remaining in contact with the valves and cooperating with the cams lobes in the course of the camshaft rotation. In order to ensure a correct performance in the valve opening and closure phases, it is necessary that the clearance existing between the base circle of each cam and the related bucket type tappet be determined in an accurate way.
In fact, if on the one hand no clearance, or an extremely limited amount of clearance, would not allow the proper closure of the valves, on the other hand an excessive clearance would detrimentally affect the performance and the life of the engine and, among other things, increase noise.
In order to attain, for each single cam/bucket type tappet coupling the required amount of clearance, generally there is foreseen the insertion--in a suitable bucket type tappet recess, at the cam or valve side--of a specifically thick adjustment shim so that the clearance between the base circle of the cam and the bucket type tappet (or the shim) be of the desired value. According to a variant, that does not involve the insertion of shims, there can be foreseen, for each cam/valve coupling, the selection of an appropriate bucket type tappet among a series of bucket type tappets that have different predetermined thicknesses. The English abstract of Japanese patent application JP-A-57013205 shows a device for calculating the gaps existing between the top faces of the valve lifters 3 and the base circles of the relevant cams of a camshaft when the latter is assembled to the cylinder head. The thickness of the shims to be inserted are chosen on the basis of the values of the calculated gaps and of the desired clearances. Two measurements are taken to calculate each gap. A first measurement is taken on the cylinder head, substantially corresponding to the distance between the top surface of each bucket type valve lifter and a camshaft bearing surface 9 of the cylinder head. A second measurement is taken on the camshaft, corresponding to the distance between the surfaces of the base circle of the cam and a bearing journal of the camshaft.
The calculations for determining the thickness of the adjustment shims, or that of the bucket type tappets, are troublesome due to various reasons among which the radial clearance existing between the main journals of the camshaft and the cylindrical seats of the cylinder head in which these journals are seated. This clearance is limited, but necessary for guaranteeing a correct rotation of the camshaft and allowing an appropriate lubrication. Devices like the one shown in the English abstract of JP-A-57013205 do not take into account such radial clearance in the calculation of the gaps.
A checking method presently used for determining the thicknesses foresees the use of apparatuses that check the dimensions of the cylinder head and those of the camshaft separately (as shown in the above cited English abstract) and the processing of the results thus obtained by supposing that, in the course of the running of the engine and the rotating of the camshaft, the main journals of the latter--urged by the thrust of the valve springs alternatively compressed by the various cams--are in constant contact with the associated cylindrical seats, at diametrally opposite positions with respect to the valves. This assumption is an approximation that depends, among other things, on the number and the angular position of the cams on the camshaft and does not guarantee highly reliable results.
In order to improve the method reliability, the results can be compensated in an empiric way, on the basis of statistics on errors detected in the course of subsequent checkings, for example when the selected shims (or the bucket type tappets) have been inserted and the camshaft is mounted in the cylinder head. In any case, this is not a really practical method of operating, since there is the need to collect an enormous amount of data and process them in an appropriate way, hence involves a considerable amount of time, high costs and not always achieves satisfying results.